Transmit Architecture for Wireless Multi-Mode Applications

ABSTRACT

In one embodiment, an apparatus includes an upconversion unit configured to upconvert a baseband signal to a radio frequency (RF) signal. A plurality of baluns for a plurality of wireless bands are provided. Multiplexing circuitry is coupled to the plurality of baluns where the upconversion unit is coupled to each balun through the multiplexing circuitry. The multiplexing circuitry is configured to multiplex the radio frequency signal from the upconversion unit to one of the plurality of baluns based on a wireless band being used.

CROSS REFERENCE TO RELATED APPLICATIONS

The present disclosure claims priority to U.S. Provisional App. No.61/179,593 for “Transmit Architecture for Cellular Multi-ModeApplications” filed May 19, 2009, U.S. Provisional App. No. 61/179,596for “Transmit Upconversion Circuitry for Cellular Multi-ModeApplications” filed May 19, 2009, and U.S. Provisional App. No.61/181,219 for “Transmit Upconversion Circuitry for Cellular Multi-ModeApplications” filed May 26, 2009, the contents of which are incorporatedherein by reference in their entirety.

BACKGROUND

Particular embodiments generally relate to wireless transmitters.

Unless otherwise indicated herein, the approaches described in thissection are not prior art to the claims in this application and are notadmitted to be prior art by inclusion in this section.

FIG. 1 depicts a conventional transmitter 100. Differential in phase (I)and quadrature (Q) signals may be processed through separate channels.For example, the I and Q signals are input into digital programmablegain amplifiers (DPGAs) 102 a/102 b, which amplify the signals.Digital-to-analog converters (DACs) 104 a/104 b convert the digital Iand Q signals to analog. The I and Q signals are then input into lowpass filters (LPFs) 106 a/106 b, which provide attenuation of componentnoise, quantitization noise, and also provide gain.

Upconverters 108 a/108 b receive the I and Q signals from low passfilters (LPFs) 106 a/106 b. Also, a synthesizer 110 generates a localoscillator (LO) signal. Frequency divider/LO generator 112 thengenerates the I version for the LO signal (LO I) and Q version for theLO signal (LO Q). The LO I signal is sent to upconverter 108 a and theLO Q signal is sent to upconverter 108 b. Upconverters 108 a and 108 bupconvert the I and Q signals (at the baseband) to differential radiofrequency (RF) signals. The differential RF signals output fromupconverter 108 a and upconverter 108 b are summed through currentsumming circuitry 114 and converted to a single-ended output through abalun 116.

Balun 116 outputs the RF signal to power amplifier (PA) buffers 118 (orpre-power amplifiers). Each PA buffer 118 may be used for a wirelessband, such as second generation (2G) high band (HB), 2G low band (LB),third generation (3G) HB, 3G HB/LB, and 3G LB. PA buffers 118 are usedto drive external power amplifiers that are off of an integrated chip(IC). The filtering and linearity that is required by wirelessapplications is often not sufficiently provided by low pass filters 106.

FIG. 2A depicts a conventional differential PA buffer 118. Upconverters108 a/108 b include Gm transistor pairs 202 a/202 b and mixers 204 a/204b, respectively. The baseband I and Q signals are received at Gmtransistor pairs 202 a and 202 b, respectively. Gm transistor pairs 202a/202 b convert a voltage to a current.

Mixer 204 a and mixer 204 b receive the LO I and LO Q signals,respectively, and upconvert the I and Q signals to differential RFsignals. The differential RF signals are combined in cascode transistorpair 206. The combined RF signals are then alternating current (AC)coupled through AC coupling capacitors 208 a and 208 b to PA buffer 118.For example, PA buffer 118 includes a differential pair of transistors210 and transistors 211 a and 211 b. PA buffer 118 buffers the signaland outputs a differential signal to balun 116. Balun 116 then outputsthe RF signal to a power amplifier at Pout.

FIG. 2B depicts a conventional single-ended PA buffer 118. Gm transistorpairs 202 a/202 b, mixers 204 a/204 b, and cascode transistor pair 206operate similarly as described with respect to FIG. 2A. The differentialRF signal from cascode transistor pair 206 is output to balun 116. Thesingle ended output of balun 116 is then AC coupled through an ACcoupling capacitor 208 to PA buffer 118. PA buffer 118 includes a firsttransistor 212 a and a second transistor 212 b that buffer the signal. Asingle-ended output is then output to the power amplifier.

In both examples in FIGS. 2A and 2B, PA buffers 118 add noise anddistortion, which affects the linearity of the signal.

SUMMARY

In one embodiment, an apparatus includes an upconversion unit configuredto upconvert a baseband signal to a radio frequency (RF) signal. Aplurality of baluns for a plurality of wireless bands are provided.Multiplexing circuitry is coupled to the plurality of baluns where theupconversion unit is coupled to each balun through the multiplexingcircuitry. The multiplexing circuitry is configured to multiplex theradio frequency signal from the upconversion unit to one of theplurality of baluns based on a wireless band being used.

In one embodiment, the multiplexing circuitry comprises a plurality ofsets of transistors, wherein each set is associated with a balun in theplurality of baluns.

In one embodiment, the apparatus includes a plurality of upconversionunits, wherein each of the plurality of upconversion units is coupled tothe plurality of sets of transistors.

In another embodiment, an apparatus comprises: a pole pair of a filterconfigured to filter a signal, the first pole pair including a firstpole and a second pole; a mirror buffer configured to buffer the signal;and a third pole of the filter coupled to the mirror buffer andconfigured to filter the signal buffered by the mirror buffer.

In one embodiment, the third pole of the filter comprises: a firstresistor; a second resistor; a capacitor; and switch circuitryconfigured to select one of the first resistor or the second resistor.

In one embodiment, the switch circuitry comprises: a first transistorcoupled to a first switch; a second transistor coupled to a secondswitch; wherein when the first switch is closed, the second switch isout of a first signal path coupling the pole pair to the first resistor,wherein when the second switch is closed, the first switch is out of asecond signal path coupling the pole pair to the second resistor.

In another embodiment, a method comprises: upconverting a basebandsignal to a radio frequency (RF) signal; determining a wireless band ina plurality wireless bands for a multimode transmitter; and multiplexingthe RF signal to a balun in a plurality of baluns based on a wirelessband being used.

The following detailed description and accompanying drawings provide abetter understanding of the nature and advantages of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a conventional transmitter.

FIG. 2A depicts a conventional differential PA buffer.

FIG. 2B depicts a conventional single-ended PA buffer.

FIG. 3 depicts an example of a transmitter according to one embodiment.

FIG. 4A depicts an example of a single ended output according to oneembodiment.

FIG. 4B shows an example of a differential output according to oneembodiment.

FIG. 4C shows another example of the transmitter for driving multiplewireless bands according to one embodiment.

FIG. 5 depicts an example of the transmitter for providing reduced poweraccording to one embodiment.

FIG. 6 depicts an example of the transmitter for providing multiplexingaccording to one embodiment.

FIG. 7 shows an example of the transmitter of FIG. 6 showing multiplewireless bands according to one embodiment.

FIG. 8A depicts an example of the transmitter showing a filter accordingto one embodiment.

FIG. 8B shows an example of the filter according to one embodiment.

FIG. 8C depicts an example of the low pass filters and filters accordingto one embodiment.

FIG. 9 depicts a more detailed example of the filter according to oneembodiment.

FIG. 10 depicts an example of the transmitter for the 2G wireless bandaccording to one embodiment.

FIG. 11 depicts a method for transmitting a signal according to oneembodiment.

DETAILED DESCRIPTION

Described herein are techniques for a wireless transmitter. In thefollowing description, for purposes of explanation, numerous examplesand specific details are set forth in order to provide a thoroughunderstanding of embodiments of the present invention. Particularembodiments as defined by the claims may include some or all of thefeatures in these examples alone or in combination with other featuresdescribed below, and may further include modifications and equivalentsof the features and concepts described herein.

FIG. 3 depicts an example of a transmitter 300 according to oneembodiment. Differential in phase (I) and quadrature (Q) signals may beprocessed through separate channels. The I and Q channels will bedescribed together but follow the paths as shown in FIG. 3. For example,the I and Q signals are amplified in digital programmable gainamplifiers (DPGAs) 302 a/302 b. Digital-to-analog converters (DACs) 304a/304 b convert the digital I and Q signals to analog. The analog I andQ signals are then input into low pass filters (LPFs) 306 a/306 b. Novelfiltering methods will be described in more detail below.

Mirror buffers 308 a/308 b receive the I and Q signals from low passfilters (LPFs) 306 a/306 b and mirror the voltage to upconverter 309.Upconverters 309 include up conversion units 310, which include basebandGm transistor pairs 312 a/312 b and mixers 314 a/314 b. Baseband Gmtransistor pairs 312 a/312 b receive the baseband I signal and thebaseband Q signal, respectively, and mirror the current from LPFs 306a/306 b. For example, baseband Gm transistor pairs 312 a/312 b convert avoltage from mirror buffers 308 a/308 b to the current. Also, asynthesizer 316 generates a local oscillator (LO) signal. Frequencydivider/LO generator 318 then generates the I version for the LO signal(LO I) and Q version for the LO signal (LO Q). The LO I and LO Q signalsmay be differential. Mixers 314 a and 314 b use the LO I signal and theLO Q signal to upconvert the baseband I and Q signals to differentialradio frequency (RF) signals.

Multiplexing circuitry 320 is provided to multiplex the RF signal tobaluns 324. Cascode multiplexers may be used to multiplex the RF signal.In one embodiment, the cascode multiplexers may be implemented usingcascode transistor pairs 322 that sum the current from mixers 314 a and314 b. For example, the radio frequency signals from mixers 304 a and304 b are combined by cascode transistor pairs 322 into a differentialradio frequency signal.

Transmitter 300 may be multimode and can transmit signals using multiplewireless bands. Wireless bands correspond to different wirelessstandards and transmit RF signals at different frequencies. Baluns 324are provided for different wireless bands, such as a second generation(2G) high band (HB), a 2G low band (LB), a third generation (3G) HB, a3G HB/LB, and a 3G LB. A balun 324 is selected based on which wirelessband is used. In this example, baluns 324 send a single-ended output toa power amplifier that is off an integrated circuit (IC), but differentoutputs may be provided. Additionally, baluns 324 may be off chip or inthe package of the PA.

Accordingly, transmitter 300 offers a direct up and out approach thatdoes not include a PA buffer. That is, balun 324 directly outputs the RFsignal off the chip without going through a PA buffer. Thus, noadditional noise and distortion is added from the PA buffer.

The same architecture for transmitter 300 can be used to drive multiplewireless bands. For example, some bands may require differential outputsand some may require single-ended outputs (balanced or unbalancedoutputs). In one embodiment, a single-ended output may be converted tomultiple differential outputs with a package change. That is, thesingle-ended output may be converted by changing a ground from aterminal of balun 324 to output a second signal. This outputsdifferential signals from the two terminals of balun 324. Thus, the samearchitecture can be configured to drive balanced or unbalanced outputs.

FIG. 4A depicts an example of a single ended output according to oneembodiment. The single ended output is simplified by showing only onebalun 324 for a wireless band. Similar structures for other wirelessbands may be provided. As shown, balun 324 outputs a single ended output(Pout) from a first terminal P. A second terminal G is coupled to ground(VSS).

The input voltage VDD may be approximately 1.8 volts. A maximum outputpower (Pout) may be met by biasing transistors as follows: baseband Gmtransistor pairs 312 a/312 b are biased in saturation, transistorsimplemented in mixers 314 a/314 b are biased in a triode region, andcascode transistor pair 322 are biased in saturation. The 3G wirelessband may require a Pout of +6 dBm (50 ohm load) with a 1.8 volt supply.These requirements are met with a better than −38 dBc adjacent channelleakage ratio (ACLR) using the above biasing approach. Conventionally,transistors in mixers 314 a/314 b are biased in saturation. However,this causes impedance change looking into cascode transistor pair 322,which affects the linearity of the signal. By biasing mixer transistorsin the triode region, the impedance change is minimal and linearity isminimally affected.

FIG. 4B shows an example of a differential output according to oneembodiment. As shown, balun 324 provides a differential output at afirst terminal P1 and a second terminal P2. Instead of having terminalP2 coupled to ground, terminal P2 outputs a signal that may be acomplementary signal of a signal output from terminal P1. Thedifferential output is used to drive a multi-mode power amplifier 410,which is off chip. As discussed above, the differential signal does notgo through a PA buffer when sent from balun 324 to power amplifier 410.

FIG. 4C shows another example of transmitter 300 for driving multiplewireless bands according to one embodiment. A first balun 324 a providesa differential output signal Pout_HB for a highband wireless band and asecond balun 324 b provides a differential output signal Pout_LB for alowband wireless band. A multi-mode/multi-band power amplifier (PA) 410receives the signals Pout_HB and Pout_LB.

In one embodiment, the power used by transmitter 300 may be reduced.FIG. 5 depicts an example of transmitter 300 for providing reduced poweraccording to one embodiment. As shown, a plurality of upconverters 309are provided in parallel. Each upconverter 309 includes an upconversionunit 310 and multiplexing circuitry 320 represented by cascodetransistor pairs 322.

N wireless bands are provided that can output a signal to one or morepower amplifiers. For example, N PAs may be provided for N wirelessbands. Although not shown for N wireless bands, a balun 324 is providedfor each wireless band and sends the RF signal to a power amplifier forthat wireless band. Each upconverter 309 is coupled to a balun 324 foreach wireless band. For example, cascode transistor pairs 322 arecoupled in parallel to each balun 324.

To reduce power, upconverters 309 may be turned off For example, cascodetransistor pairs 322 may be turned off by biasing the gates of cascodetransistor pairs 322 with a voltage to turn them off This also reducesthe current used by transmitter 300 because current does not flowthrough cascode transistor pairs 322 when they are turned off Thus, thecurrent and power may be reduced by turning off a percentage ofupconverters 309. For example, if half of cascode transistor pairs 322are turned off for half of upconverters 309, then the power and currentmay be reduced by half.

In addition to being able to reduce power, multiplexing circuitry 320may also be used to couple upconversion units 310 to different wirelessbands. For example, FIG. 6 depicts an example of transmitter 300 forproviding multiplexing according to one embodiment. An N number ofupconversion units 310 are provided. For example, 80 upconversion units310 may be provided. Upconversion units 310 receive I and Q signals,upconvert the I and Q signals to RF signals, and then output the RFsignals to multiplexing circuitry 320. The implementation shown in FIG.6 is shown for a single mixer 314 a. Similar circuitry may be used formixer 314 b and combined with the implementation shown in FIG. 6.

Multiplexing circuitry 320 is configured to couple upconversion units310 to the wireless bands that are supported by transmitter 300. Forexample, seven wireless bands as described above may be supported. Eachwireless band includes a balun 324 that is used to output the RF signalto a power amplifier. As shown, a band #1 for 3G HB and a band #2 for 3GHB are provided. Although these two bands are shown, other wirelessbands may be provided.

Multiplexing circuitry 320 may include sets of cascode multiplexers,which multiplex the RF signal from mixers 314 among baluns 324. In oneembodiment, cascode transistor pairs 322 are used to couple upconversionunit 310 to baluns 324. Although cascode transistor pairs 322 aredescribed, other implementations may be used. In one example, the numberof sets of cascode transistor pairs 322 may be N*X, where N is thenumber of upconversion units and X is the number of outputs. Forexample, if there are seven outputs, then seven sets of 80 cascodetransistor pairs 322 are provided for a total of 560 cascode transistorpairs 322. Thus, each output includes a set of 80 cascode transistorpairs 322.

When a wireless band is selected, the set of cascode transistor pairs322 associated with that wireless band are turned on and cascodetransistor pairs 322 for the other wireless bands are turned off. Forexample, if band #1 is used, a bias voltage Band1_ON at the gates ofcascode transistor pairs 322 a is biased such that cascode transistorpairs 322 a turn on. Also, a bias voltage Band2_ON is biased at thegates of cascode transistor pairs 322 b such that the set of cascodetransistor pairs 322 b for wireless band #2 are turned off This couplesupconversion units 310 to balun 324 a through cascode transistor pairs322 a. Also, since cascode transistor pairs 322 b are off, upconversionunits 310 are not coupled to balun 324 b.

Conversely, when wireless band #2 is used, then the bias voltageBand2_ON is biased such that cascode transistor pairs 322 b are turnedon while the bias voltage Band1_ON is biased such that cascodetransistor pairs 322 a are turned off This couples upconversion units310 to balun 324 b and not balun 324 a. Accordingly, multiplexing isprovided by biasing different sets of cascode transistor pairs 322 toturn on one set and turn the other sets off.

FIG. 7 shows an example of transmitter 300 of FIG. 6 showing multiplewireless bands according to one embodiment. In the example, a singleupconversion unit 310 for the I channel and Q channel is shown. Multipleupconversion units 310 may be connected in parallel, but are not shown.

Upconversion unit 310 is coupled to multiplexing circuitry 320. Asimplified schematic of multiplexing circuitry 320 is provided in thatall cascade transistor pairs 322 in a set for a balun 324 are not shown.In the simplified schematic, cascode transistor pairs 322 coupleupconversion unit 310 to baluns 324 a-324 f. The six output baluns 324each provide a signal for a different wireless band to a poweramplifier. A seventh output (not shown) may be coupled to resistors andto the supply voltage VDD for offset and gain calibration. Multiplexingcircuitry 320 multiplexes the RF signal from upconversion unit 310 tobalun 324 based on which wireless band is selected. For example, if awireless band 3G HB is selected, then upconversion unit 310 is coupledto balun 324 d through cascode transistor pairs 322 for balun 324 d.

A filter may be inserted in between mirror buffer 308 and upconversionunits 310 to provide filtering of I and Q signals. FIG. 8A depicts anexample of transmitter 300 showing filters 802 a/802 b according to oneembodiment. Filters 802 a and 802 b are provided in between mirrorbuffers 308 a/308 b and upconversion unit 310. Filters 802 a/802 binclude resistors 812 a/802 b and capacitors 814 a/814 b, respectively.FIG. 8 is a simplified view of one upconversion unit 310. However, otherupconversion units may be provided in parallel.

Filters 802 a/802 b provide filtering of noise from low pass filter 306and also from mirror buffer 308. For example, signals Vg1 p and Vg1 nfor the I channel are filtered by filter 802 a and filtering of signalsVg2 p and Vg2 n for the Q channel are filtered through filter 802 b. Forboth the I and Q signals, the outputs of low pass filters 306 a/306 bare converted to a current using resistors 806 a/806 b (RGm). Resistors806 a/806 b are at virtual ground due to the negative feedback from thedrains of transistors 810 a/810 b, respectively, and convert a voltagefrom the output of low pass filters 306 a/306 b to a current.

The current is then input into differential op amps 808 a and 808 b,respectively. Transistors 810 a and 810 b are coupled to the output ofop amps 808 a/808 b, respectively, and have their gates modulated bysignals Vg1 p/Vg1 n and Vg2 p/Vg2 n, respectively, at a voltage thatcreates a drain current equal to the current flowing through each ofresistors 806 a/806 b. The signals Vg1 p and Vg1 n pass through filter802 a and then modulate the gate of Gm transistor pairs 312 a. Thesignals Vg2 p and Vg2 n pass through filter 802 b and then modulate thegates of Gm transistor pairs 312 b. This creates an AC currentproportional to the output voltage from low pass filters 306 a/306 b atthe drains of Gm transistor pair pairs 312 a/312 b, respectively.

Filters 802 a/802 b may be implemented differently as shown in FIG. 8A.For example, FIG. 8B shows an example of filter 802 a according to oneembodiment. Filter 802 b may be similarly implemented. A first filter1-802 a is coupled to receive the signal Vg1 p and includes a resistor1-812 a and a capacitor 1-814 a. Capacitor 1-814 a is coupled to ground.The output of filter 1-802 a is coupled to a first transistor of Gmtransistor pair 312 a.

A filter 2-802 a receives the signal Vg1 n. A resistor 2-812 a andcapacitor 2-814 a are provided. Capacitor 2-814 a is coupled to ground.The output of filter 2-802 a is coupled to a second transistor of Gmtransistor pair 312 a. Filters 1-802 a and 2-802 a may be used in lieuof filter 802 shown in FIG. 8A.

Referring back to FIG. 8A, filters 802 a/802 b may be part of an nthorder filter when combined with low pass filter 306. For example, lowpass filter 306 and filter 802 may be part of a third or fifth orderfilter. FIG. 8C depicts an example of low pass filters 306 a/306 b andfilters 802 a/802 b according to one embodiment.

Low pass filters 306 a/306 b may be implemented as one or two complexpole pairs (CPPs). As shown, two CPPs 854 a/854 b and 856 a/856 b areprovided in this example. The poles of the filter may be determinedbased on the transfer function of the filter. In one embodiment, aButterworth filter is implemented using low pass filter 306 and filter802; however, other filters may be used.

CPPs 1 provide filtering of noise introduced by DACs 304 a/304 b,respectively. CPPs 2 also provide filtering of noise introduced by DACs304 a/304 b and CPP 1. Filters 802 a/802 b provide filtering of noiseintroduced by CPP2 and also mirror buffers 308 a/308 b, respectively.

Filters 802 a/802 b may be the fifth pole of a 5^(th) order filter,which implements a fifth order response filter, or the third pole of a3^(rd) order filter, which implements a third order response filter.Filters 802 a/802 b may be referred to as a Tx pole. Although a third orfifth order filter is described, an Nth order filter may also be used.Because filters 802 a/802 b are in a signal path with CPP1 and CPP2,respectively, filters 802 a/802 b are a real pole in the higher orderfilter. In one embodiment, the Tx pole is placed at a frequency highenough away from the band edge of the baseband spectrum so thatlinearity is minimally impacted but meaningful filtering of noisecontributions of noise is provided. For example, if baseband signal isat 2 MHz, the Tx pole may be placed around 4 MHz or 2 times thefrequency of the baseband signal.

Filter 802 may be programmable. FIG. 9 depicts a more detailed exampleof filter 802 a according to one embodiment. Only one branch of the Isignal processing channel is shown as noted by the axis of symmetry.Other branches of the I channel and the Q channel operate similarly.Filter 802 a is programmable in that a first resistor 858 a or a secondresistor 858 b may be coupled to the output of low pass filter 306 a.Additional resistors 858 may also be used.

Filter 802 a provides programmability and also limits distortion. Forexample, switch resistance may cause the nonlinearity. The distortion islimited because switches for resistor 858 a are not in the signal pathwhen resistor 858 b is selected and switches for resistor 858 b are notin the signal path when resistor 858 a is selected.

When resistor R1 is desired for the Tx pole, the voltage S1 is high(e.g., 1) and the voltage S1B is low (e.g., 0). This closes a switch(sw1 a) 860 a, which couples a transistor (M1) 861 b to the LPF outputthrough resistor 806 a and turns transistor M1 on. The voltage S1B opensa switch (sw1 b) 860 b. Also, a switch (sw1 c) 860 c is closed becausethe voltage S1 is high. This couples the voltage Vc1 to the gate of atransistor (M1C) 861 a and biases transistor M1C in the saturationregion (the biasing can also be the triode region) such that transistorM1C is turned on. Also, the voltage S2 is low (e.g., 0) then the voltageS2 b is high (e.g., 1). This opens a switch (sw2 a) 862 a and closes aswitch (sw2 b) 862 b. Also, S2 is low and a switch 862 c is open. Thiscouples a transistor (M2C) 866 a to ground (VSS) and transistor M2C isnot on.

The above puts transistor M1 and transistor M1C in a feedback loop andselects resistor R1 for the loop filter resistor. The switchnon-linearity of switch sw1 a is in the feedback loop and the feedbackloop compensates for any non-linearity caused by the switch. Also,because switch 862 a is open, the gate of a transistor 866 b (M2) isfloating. Thus, resistor R2 is out of the circuit. Further, the signalpath from transistor M1 to the output of filter 802 a does not includeany switches and thus results in a highly linear signal.

When resistor (R2) 858 b is selected, the voltage S1 is low and thevoltage S1 b is high. This couples the gate of transistor M1C to groundand thus it is not turned on. Switch 860 a is also opened such thattransistor M1 is not coupled to the low pass filter output.

The voltage S2 is high and the voltage S2 b is low. This closes switch862 a and biases a transistor (M2) 866 b to turn on. Also, transistorM2C is coupled to the voltage Vc2 and is biased to turn on. Thisprovides a signal path that selects resistor R2. The signal path fromtransistor M2 to the output of filter 802 a does not include a switchand thus results in a highly linear signal. Thus, a highly linearprogrammable filter is provided.

Transmitter 300 may also be used to support the 2G wireless band. FIG.10 depicts an example of transmitter 300 for the 2G wireless bandaccording to one embodiment. The 2G band may use only the I channel.Thus, the operation of transmitter 300 may be altered to not use the Qchannel. A direct current (DC) output is output by DAC 304 a in the Ichannel. A zero output is output in the Q channel from DAC 304 b. DACs304 a and 304 b may be programmed to output the DC output and the zerooutput.

Synthesizer 316 generates a Gaussian minimum shift keying (GMSK) signal.The GMSK signal is used to upconvert the I signal to a 2G radiofrequency signal. Frequency divider/LO generator 318 generates the LO Iand LO Q modulating signals. The I signal in the I channel is mixed withthe LO I signal at mixer 314 a. The Q channel has been disabled with thezero output and the LO Q signal is then output by mixer 314 b. However,cascode transistor pairs 322 b are disabled by biasing their gates toVSS. Thus, this channel is off or disconnected from balun 324. Mixer 314a upconverts the I signal to an RF signal using the GMSK LO I signal.The gates of cascode transistor pairs 322 a are biased with a voltageVc1 that biases the transistors in the saturation region to turn thetransistors on. Thus, the RF signal output by mixer 314 a is sent tobalun 324.

Accordingly, the same I channel and Q channel path may be used for the2G and 3G wireless bands. However, as discussed with respect to FIG. 10,the 2G band uses the DC output applied to the I channel and the Qchannel is turned off When a wireless band that requires both channelsis being used, such as 3G, both channels may be enabled.

FIG. 11 depicts a method for transmitting a signal according to oneembodiment. At 1102, I and Q signals are amplified. At 1104, the I and Qsignals are converted to analog I and Q signals. At 1106, filtering isapplied to the I and Q signals.

At 1108, the I and Q signals are upconverted to RF signals. At 1110,multiplexing for a wireless band is performed. At 1112, the RF signalsare sent to a balun 324 for output to a power amplifier.

As used in the description herein and throughout the claims that follow,“a”, “an”, and “the” includes plural references unless the contextclearly dictates otherwise. Also, as used in the description herein andthroughout the claims that follow, the meaning of “in” includes “in” and“on” unless the context clearly dictates otherwise.

The above description illustrates various embodiments of the presentinvention along with examples of how aspects of the present inventionmay be implemented. The above examples and embodiments should not bedeemed to be the only embodiments, and are presented to illustrate theflexibility and advantages of the present invention as defined by thefollowing claims. Based on the above disclosure and the followingclaims, other arrangements, embodiments, implementations and equivalentsmay be employed without departing from the scope of the invention asdefined by the claims.

1. An apparatus comprising: an upconversion unit configured to upconverta baseband signal to a radio frequency (RF) signal; a plurality ofbaluns for a plurality of wireless bands; and multiplexing circuitrycoupled to the plurality of baluns, wherein the upconversion unit iscoupled to each balun through the multiplexing circuitry, and whereinthe multiplexing circuitry is configured to multiplex the radiofrequency signal from the upconversion unit to one of the plurality ofbaluns based on a wireless band being used.
 2. The apparatus of claim 1,wherein multiplexing circuitry comprises a plurality of sets oftransistors, wherein each set is associated with a balun in theplurality of baluns.
 3. The apparatus of claim 2, further comprising aplurality of upconversion units, wherein each of the plurality ofupconversion units is coupled to the plurality of sets of transistors.4. The apparatus of claim 3, wherein, through the multiplexingcircuitry, one set of transistors are coupled to one of the plurality ofbaluns to send the radio frequency signal from the upconversion unitthrough the one of the plurality of baluns.
 5. The apparatus of claim 4,wherein the one set of transistors is coupled to the one of theplurality of baluns by biasing the one set of transistors to be on andbiasing the other sets of transistors to be off.
 6. The apparatus ofclaim 1, wherein the upconversion unit comprises: a Gm transistor biasedin a saturation region; and a mixer transistor biased in a trioderegion, wherein the multiplexing circuitry comprises a multiplexingtransistor biased in the saturation region.
 7. The apparatus of claim 1,wherein the one of the plurality of baluns comprises a differentialoutput or a single-ended output.
 8. The apparatus of claim 1, furthercomprising: a filter; and a mirror buffer, wherein a pole of the filteris coupled in between the mirror buffer and the upconversion unit. 9.The apparatus of claim 8, wherein the filter comprises a pair oftransmitter poles coupled to an input of the mirror buffer.
 10. Theapparatus of claim 1, wherein the upconversion unit comprises: a firstmixer configured to mix an in phase (I) signal with a first localoscillator signal to generate a first radio frequency signal; and asecond mixer configured to mix a quadrature (Q) signal with a secondlocal oscillator signal to generate a second radio frequency signal,wherein the multiplexing circuitry is configured to couple the firstmixer to the one of the baluns and not couple the second mixer to theone of the baluns.
 11. An apparatus comprising: a pole pair of a filterconfigured to filter a signal, the first pole pair including a firstpole and a second pole; a mirror buffer configured to buffer the signal;and a third pole of the filter coupled to the mirror buffer andconfigured to filter the signal buffered by the mirror buffer.
 12. Theapparatus of claim 11, wherein the pole pair comprises a first polepair, wherein the apparatus further comprises a second pole pairincluding fourth and fifth transmitter poles coupled to the first polepair.
 13. The apparatus of claim 11, wherein the third pole of thefilter comprises: a first resistor; a second resistor; a capacitor; andswitch circuitry configured to select one of the first resistor or thesecond resistor.
 14. The apparatus of claim 13, wherein the switchcircuitry comprises: a first transistor coupled to a first switch; asecond transistor coupled to a second switch; wherein when the firstswitch is closed, the second switch is out of a first signal pathcoupling the pole pair to the first resistor, wherein when the secondswitch is closed, the first switch is out of a second signal pathcoupling the pole pair to the second resistor.
 15. The apparatus ofclaim 13, wherein the switch circuitry comprises: a first transistorcoupled to a first switch, the first switch coupled to the pole pair; asecond transistor coupled to: a second switch coupled to a voltagesupply; and a third switch coupled to ground; a third transistor coupledto a fourth switch, the fourth switch coupled to the pole pair; a fourthtransistor coupled to: a fifth switch coupled to a voltage supply; and asixth switch coupled to ground.
 16. A method comprising: upconverting abaseband signal to a radio frequency (RF) signal; determining a wirelessband in a plurality wireless bands for a multimode transmitter; andmultiplexing the RF signal to a balun in a plurality of baluns based ona wireless band being used.
 17. The method of claim 16, whereinmultiplexing comprises coupling the RF signal through a set oftransistors in a plurality of sets of transistors, wherein each set oftransistors is associated with one balun in the plurality of baluns. 18.The method of claim 17, wherein coupling comprises biasing the one setof transistors to be on and biasing the other sets of transistors to beoff.
 19. The method of claim 16, further comprising: filtering thebaseband signal using a first filter; buffering the baseband signalusing a mirror circuit; and filtering the baseband signal from themirror circuit using a second filter.
 20. The method of claim 19,wherein the first filter and the second filter form an Nth order filter.